1. Field of the Invention
The present invention relates to a glass-coated thick film resistor coated by a crystallizable glass having a firing temperature of 400.degree. to 600.degree. C. and consisting of PbO, ZnO, B.sub.2 O.sub.3, Al.sub.2 O.sub.3 and SiO.sub.2.
2. Description of the Prior Art
In general, a glass-coated thick film resistor has heretofore been obtained by printing a conductive paste onto an alumina substrate, firing the coating to form at least two terminals, printing a resistor paste consisting of conductive powder such as Ag-Pd or RuO.sub.2, glass frit and an organic vehicle onto said substrate and said terminals, firing the coating to form a thick film resistor, printing a glass paste to cover the resistor completely, and then firing the coating to form a glass-coated thick film resistor.
As for the characteristic required for the glass used in the formation of said glass coating, it is necessary for the glass to have a firing temperature of 400.degree. to 600.degree. C. If the firing temperature is lower than 400.degree. C., Ag in Ag-Pd which is a component of the conductor is undesirably oxidized, resulting in an increase in the resistance of a conductor part and the deterioration of solderability. Also, if the firing temperature is higher than 600.degree. C., the resistance of the resistor undesirably increases. When the firing temperature is 400.degree. C. or more, oxide of Ag in Ag-Pd which is a component of the conductor is decomposed.
Amorphous glasses such as lead borosilicate glass have heretofore been used in the glass coating. However, the low melting amorphous glasses have a defect in that their water resistance is poor. In order to obviate this defect, a crystallized glass consisting of PbO, ZnO and B.sub.2 O.sub.3 was examined, but it was found that the glass had a defect in that its water resistance was poor.